1. Field of the Invention
This invention relates a surface-emitting laser having a first distributed Bragg reflector composed of two kinds of thin film stacked alternately, an active layer and a second distributed Bragg reflector composed of two kinds of thin film stacked alternately which are formed on a semiconductor substrate successively.
2. Description of the Prior Art
Recently, a diode or a transistor widely used as a semiconductor element has a metal-oxide-semiconductor structure composed of a semiconductor layer, an insulating film obtained by oxidizing an arsenic aluminum (AlAs) layer formed on the semiconductor layer and a metallic film formed on the insulating film. The technique is suggested by universities in the United Stated, etc that by applying the manufacturing method of metaloxide-semiconductor structure for a surface-emitting laser, the AlAs layer or the gallium arsenic aluminum (GaAlAs) layer constituting the structure is selectively oxidized from its sides and thereby, a current path is restricted and the current is flown in a local area. It is known that the surface-emitting laser produced by applying the technique has enhanced characteristics in operation current, operation efficiency, etc.
FIGS. 1 and 2 are principle structures of conventional examples in a surface-emitting laser having the above selective oxidation structure, respectively.
A surface-emitting laser shown in FIG. 1 has a semiconductor substrate 51, a first distributed Bragg reflector 54, an active layer 55 and a second distributed Bragg reflector 56. The first and the second reflectors are composed of alternately stacked GaAs layers 52 and GaAlAs layers 53, respectively. Moreover, between at least one of the reflectors 54, 56 (the reflector 56 in the figure) and the active layer 55 is provided a current stenosed layer 57 formed by oxidizing a given area in a remote junction surface of an AlAs layer 58, which is formed between the reflector 56 and the active layer 55. In this example, the current stenosed layer 57 restricts a current path.
A surface-emitting laser shown in FIG. 2 has a semiconductor substrate 51, a first distributed Bragg reflector 54, an active layer 55 and a second distributed Bragg reflector 56. The first and the second reflectors are composed of alternately stacked GaAs layers 52 and AlAs layers 53, respectively. Moreover, the whole stacked structure, including the reflectors 54, 56, is oxidized. In this example, the nearest one of the plural oxidized AlAs layers to the active layer 55 serves as a current-stenosed layer and restricts a current path. The plural oxidized AlAs layers near the active layer reduce the capacitance of the surface-emitting laser itself to some degree.
The conventional surface-emitting laser shown in FIG. 1 can restrict the current path by the current stenosed layer 57 formed on the active layer 55, but the above structure increases the capacitance of the surface-emitting laser itself including the structure. Thus, in trying to operate the laser at a high speed, the laser has difficulty in response to external signals, so that its high speed operation can not be realized.
Moreover, it is conceivable that by forming a current blocking layer through implantation protons in the above structure, the capacitance of the surface-emitting laser is reduced. In this case, however, a processing equipment exclusively for the proton-implantation is needed, resulting in the increase in cost. Furthermore, semiconductor layers constituting the distributed Bragg reflector suffer from the proton-implantation, resulting in the occurrence of defect in the layers and it is difficult to control a implantation position in a thickness direction of the above structure. Accordingly, this implantation method has difficulty in being actually adopted.
On the other hand, the surface-emitting laser shown in FIG. 2 can restrict a current path by the plural oxidized AlAs layers and reduce the capacitance of the laser to some degree. However, a light generated in the surface-emitting laser is scattered at the boundary between the oxidized area and the non-oxidized area in the adjacent AlAs layer to the one serving as the current stenosed layer. Accordingly, the emitting efficiency of the light emitted to outside from the surface-emitting laser is degraded, resulting in the deterioration of the laser""s performance and it is very difficult to emit the light having a xe2x80x9csingle mode peakxe2x80x9d.
It is an object of the present invention to iron out the above problems by providing, in a surface-emitting laser, a capacitance reducing layer able to reduce the capacitance of the laser without scattering of a light emitted to outside.
The first invention claimed in claim 1 relates to a surface-emitting laser in which a first distributed Bragg reflector composed of an alternately stacked structure made of two kinds of thin film, an active layer and a second distributed Bragg reflector composed of an alternately stacked structure made of two kinds of thin film, are formed on a semiconductor substrate, successively, comprising a current stenosed layer having an oxidized area in a remote junction surface therein between at least one of the first and the second distributed Bragg reflectors and the active layer, and plural capacitance-reducing layers, each layer having a smaller oxidized area than the oxidized area in a remote junction surface constituting the current stenosed layer, at least one of the first and the second distributed Bragg reflectors, the plural capacitance-reducing layers, the current stenosed layer and the active layer being arranged successively, one of the first and the second distributed Bragg reflectors constituting a first conductive type Bragg reflector, the other constituting a second conductive type Bragg reflector.
The second invention claimed in claim 2 relates to the surface-emitting laser in which the first and the second distributed Bragg reflectors are composed of alternately stacked GaxAl1xe2x88x92xAs (herein, 0xe2x89xa6xxe2x89xa61) layers and GayAl1xe2x88x92yAs (herein, 0xe2x89xa6yxe2x89xa61).
The third invention claimed in claim 3 relates to the surface-emitting laser in which the current stenosed layer is composed of the plural semiconductor layers, the semiconductor layer at its junction surface being composed of an AlAs layer having a thickness of 10-30 nm and the capacitance-reducing layer is composed of a GaAlAs layer having a thickness of about 80 nm, and total thickness between the first and the second distributed Bragg reflectors is one-fourth of the emitting wavelength from the surface-emitting laser.
The fourth invention claimed in claim 4 relates to the surface-emitting laser in which the current stenosed layer is composed of an AlAs layer and the capacitance-reducing layer is composed of an AlAs layer having a thinner thickness than that of the current stenosed layer.
In the first invention, when an anode electrode and a cathode electrode are provided on the second distributed Bragg reflector and the semiconductor substrate, respectively, an injected current flows only through the non-oxidized area of the current stenosed layer and reaches the local part of the active layer because the oxidized area formed except the center of the remote junction surface from the active layer in the semiconductor constituting the stenosed layer is insulative and does not flow the current through itself. As a result, the part of the active layer corresponding to the non-oxidized area is selectively excited and generates a light, which is amplified by the reflection between the first and the second distributed Bragg reflectors and is emitted. The generated light is scattered at the boundary between the oxidized area and the non-oxidized area in the semiconductor layer constituting the current stenosed layer. However, since the capacitance-reducing layer, adjacent to the current stenosed layer, has a larger non-oxidized area than that of the stenosed layer, the generated light does not contact the boundary between the oxidized area and the non-oxidized area in the semiconductor layer constituting the capacitance-reducing layer, resulting in the prevention of the scattering of the generated light therein. Moreover, since the stacked structure has plural capacitance-reducing layers, each layer being composed of an oxidized semiconductor layer, it exhibits a similar capacitance-reducing effect to the case that it has very thick insulating layer. Accordingly, the capacitance of the surface-emitting laser can be reduced without the scattering of the light to be emitted to outside and thereby, the surface-emitting laser able to be operated at a high speed can be realized.
In the second invention, since the first and the second distributed Bragg reflectors are composed of alternately stacked GaxAl1xe2x88x92xAs (herein, 0xe2x89xa6xxe2x89xa61) layers and GayAl1xe2x88x92yAs (herein, 0xe2x89xa6yxe2x89xa61) layers, respectively, they exhibit high reflectances.
In the third invention, since the current stenosed layer is composed of the plural semiconductor layers and the semiconductor layer at its junction surface is an AlAs layer having a thickness of 10-30 nm, the deterioration of the scattering at the boundary between the oxidized area and the non-oxidized area in the AlAs layer having a thickness of more than 30 nm can be prevented. Then, the degradation of the oxidation proceeding speed in the AlAs layer having a thickness of less than 10 nm can be prevented. Furthermore, since the capacitance-reducing layer is composed of a GaAlAs layer having a thickness of about 80 nm, which corresponds to one-fourth of the emitting wavelength xcex in the surface-emitting layer, it can satisfy the condition of maximizing the reflectance.
In the fourth invention, since the capacitance-reducing layer is composed of plural thinner AlAs layers than that in the current stenosed layer, the whole structure constituting the surface-emitting laser is composed of almost alternately stacked AlAs layers. That is, a desired laser can be made of a simple stacked structure.